mv: decide overwrite prompts by effective write access

[MuntasirSZN]

mv checked the owner write bit instead of write access, so root was prompted
before overwriting a 0555 file, and a symlink to an unwritable file drew a
spurious prompt.

is_writable now uses accessat with AT_EACCESS and treats symlink destinations as
writable, and the prompt wording comes from that same check. Tests cover both.

This failed in NixOS very badly.

[Copilot]

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[anastygnome]

@cakebaker I've implemented it in the faccessat.rs file in the test until. Maybe we should make it a common function in uucore

[anastygnome]

refs #15294

[MuntasirSZN]

@anastygnome should I close this pr?

[MuntasirSZN]

Btw your pr is better in every way (though some conflicts)

[anastygnome]

@MuntasirSZN you can close this but can you please make sure I didn't forget a case in the MV commit on my pr?

[MuntasirSZN]

@anastygnome Symlinks: is_writable still resolves the link and accessat follows it, so a symlink to an unwritable file still draws a spurious prompt; that case reproduces as a plain user. Wording: get_interactive_prompt still keys off 0o200, so the prompt text can disagree with the prompt decision.

[anastygnome]

@MuntasirSZN Please be cleare I didnt get that, do you have a clear reproducing scenario?

[MuntasirSZN]

@anastygnome

❯ mv --version
mv (GNU coreutils) 9.9
Packaged by https://nixos.org
Copyright (C) 2025 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <https://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by Mike Parker, David MacKenzie, and Jim Meyering.
❯ cd /tmp && rm -rf repro1 && mkdir repro1 && cd repro1
direnv: unloading
❯ echo OLD > real && chmod 0555 real && ln -s real link && echo NEW > src
❯ printf 'n\n' | script -qec "/tmp/opencode/mv-pr15294 src link" /dev/null
n
mv-pr15294: replace 'link', overriding mode 0555 (r-xr-xr-x)? ⏎                                                                                                                           ❯ # but gnu
❯ cd /tmp && rm -rf repro1 && mkdir repro1 && cd repro1
❯ echo OLD > real && chmod 0555 real && ln -s real link && echo NEW > src
❯ script -qec "mv src link"
❯ # no prompt!
❯ cd /tmp && rm -rf repro2 && mkdir repro2 && cd repro2 && echo NEW > src
❯ ls -l /etc/passwd
-rw-r--r-- 1 root root 4658 Oct 13 20:21 /etc/passwd
❯ printf 'n\n' | script -qec "/tmp/opencode/mv-pr15294 src /etc/passwd" /dev/null
n
mv-pr15294: overwrite '/etc/passwd'? ⏎                                                                                                                                                    ❯ # but gnu
❯ script -qec "mv src /etc/passwd"
mv: replace '/etc/passwd', overriding mode 0644 (rw-r--r--)? ^C⏎
❯ # wording is off!

[anastygnome]

OK, that's a separate issue! I'll focus on the technical aspects